In New Zealand, the current storm water maintenance program involves: street sweeping; catchpit cleaning and unblocking; storm water line cleaning and unblocking; and soakhole cleaning and repair work.
The limited ability of catchpits to remove sediment and debris results in a large amount of time and expense having to be spent on unblocking catchpits, storm water lines and soakholes to prevent flooding. The existing maintenance expenditure for the Auckland Central Business District is estimated at $900 hectare/year.
Typical storm water treatment devices, such as sedimentation ponds or sand filters, often require a large capital investment, land acquisition, and often cannot be incorporated into high urbanized.
Ground surfaces in an urban environment are contaminated by urban activities, in particular motor vehicle use. Most serious pollutants originate from motor vehicles for example exhaust emissions, brake linings, rubber heavy metals, oils, greases and complex organic compounds. In addition pollution in storm water can arise from the use of harmful biocides in agricultural, industrial and domestic applications. Also volatile solids enter the storm water system, whilst these primarily include plant materials, they also include animal faces, industrial waste, discarded food products, and other organics. These solids contain the pathogens, bacteria and nutrients that may adversely affect water quality.
A high proportion of suspended solids pass through existing drainage systems to streams or rivers and ultimately the marine environment.
Whilst primary prevention of contamination is very important it is also appropriate to consider systems for removal of pollution that does occur.
Control of contamination, in particular at the interface between a control system and the source, offers a practical and cost effective means of pollution control in urban environments.
Storm water runoff in urban environments is collected through drains which generally incorporate catchpits. Whilst catchpits are designed to retain a proportion of the sediment, their efficiency in this regard is limited, particularly at high rates of water flow and when the sediment level in the catchpit is close to the catchpit outlet.
Most catchpits in Auckland have been fitted with half siphons to prevent odors and to prevent sediment escaping into the storm water system. The half siphons which cause flow to travel vertically and make two 45 degree turns can also cause solids to be trapped and in turn the trapped solids trap more solids until the pipe is completely blocked. Blocked catchpits are air blasted and/or jetted out using a hydro blaster. However, some material may solidify in the half siphon and it becomes very difficult to completely remove this.
Occasionally the catchpit becomes blocked with concrete or cement making it impossible to unblock. When this occurs the catchpit must be dug up and replaced. These blockages can cause local flooding resulting in damaged public and private amenities. Unblocking catchpits and storm water lines requires significant maintenance expenditure each year.
A recent study performed by the Island Care New Zealand Trust estimated that 28,000 pieces of litter are discharged from Auckland City storm water systems per day.